Recently, transparent materials that are non-shattering properties or have a shattering resistance greater than glass have been widely employed in place of transparent sheet glass. For instance, since plastic substrates, particularly, carbonate resin substrates, are excellent in transparency, impact resistance, and heat resistance, they have now been used in various applications, in place of glass, including structural members such as windows of buildings and vehicles and instrument covers, and the like.
However, these materials are poorer in surface characteristics such as scratch resistance and weatherability than glass, so that there is a strong demand of improving the surface characteristics of polycarbonate resin moldings. In recent years, vehicle windows and sound-insulating walls for road have been required to withstand outdoor exposure over ten years or over.
For the improvement of weatherability of polycarbonate resin moldings, there have been proposed a method of laminating an acrylic resin film whose weatherability is excellent on the surface of a polycarbonate resin substrate and a method wherein a resin layer containing a UV absorber on the resin surface is provided by co-extrusion or the like.
Moreover, the methods of improving a scratch resistance of polycarbonate resin moldings have been proposed including a method wherein a thermosetting resin such as a polyorganosiloxane or melamine-based resin is coated thereon, and a method of coating a polyfunctional acrylic, photocurable resin.
On the other hand, the manufacture of transparent bodies having both good weatherability and scratch resistance has been known as described, for example, in JP-A 56-92059 and JP-A 1-149878 (referred to as Patent Documents 1 and 2, hereinafter), from which there is known a UV absorbing, transparent substrate wherein a protective film of a colloidal silica-containing polysiloxane coating composition is formed through a primer layer to which a large amount of a UV absorber is added.
However, the addition of a large amount of a UV absorber to the primer layer is disadvantageous in poor adhesion between the primer layer and the protective film formed from the colloidal silica-containing polysiloxane coating composition that is coated onto the upper surface of a substrate or the primer layer and also in the removal of the UV absorber from the composition during the thermal curing step, for example, by volatilization. In addition, where outdoor use is extended over a long term, the UV absorber is allowed to gradually bleed out thereby causing cracks, whitening or yellowing. Moreover, a problem is involved in that the UV absorber cannot be added in large amounts to the upper side protective film made of the colloidal silica-containing polysiloxane from the standpoint of the scratch resistance.
It is known, for example, in JP-A 8-151415 (Patent Document 3) that a protective film is formed on the surface of synthetic resins using, as coating ingredients, a benzotriazole-based UV absorbing vinyl monomer or benzophenone-based UV absorbing vinyl monomer and a vinyl monomer copolymerizable therewith. However, limitation is placed on the scratch resistance because of the protective film being made of the vinyl polymer.
Further, it is also known, for example, from JP-A 2001-114841, Japanese Patent No. 3102696 and JP-A 2001-214122 and JP-A 2001-47574 (Patent Documents 4 to 7) that there can be obtained multi-layered resin laminates, which are imparted with weatherability while keeping adhesion to a resin substrate, when using as a coating ingredient, a copolymer made up of a benzotriazole-based UV absorbing vinyl monomer or benzophenone-based UV absorbing vinyl monomer, an alkoxysilyl group-containing vinyl monomer, and a vinyl monomer copolymerizable therewith.
In these references, coated products imparted with a good scratch resistance and weatherability are obtained by providing these copolymer-containing coating compositions as a primer and forming a colloidal silica-containing polysiloxane resin film on the primer layer. Although these coated products are significantly improved in adhesion to the polysiloxane resin film and weatherability, crosslinking networking of the alkoxysilyl group in the primer layer is not allowed to proceed satisfactorily. Eventually, a non-cured residual alkoxysilyl group or hydroxysilyl group undergoes after crosslinkage with time, which is liable to cause a strain in the film, with the attendant disadvantage that defects such as of cracks and peeling off are apt to occur. Thus, long-term weatherability is still unsatisfactory. Additionally, when the film is exposed to an abrupt change in ambient temperature, especially, to a change at a relatively high temperature, the cracks ascribed to the above-mentioned after-crosslinkage is inconveniently liable to occur.
Further, in JP-A 2004-1393 (Patent Document 8), adhesion and anti-cracking properties are improved by specifying a difference in linear expansion coefficient between a substrate and a primer layer and also a difference in linear expansion coefficient between the primer layer and a polysiloxane cured layer. However, a UV absorber cannot be incorporated in the primer layer in large amounts and thus, long-term weatherability has been still unsatisfactory.
We have already proposed in JP-A 2008-120986 (Patent Document 9) a primer composition which includes a copolymer of a UV absorbing vinyl monomer, an alkoxysilyl group-containing vinyl monomer and a vinyl monomer copolymerizable with these monomers, and silica fine particles.
The film obtained by coating and curing the primer composition becomes low in linear expansion coefficient owing to the dense three-dimensional crosslinking network structure formed through the siloxane crosslinkage of the hydrolysable silyl group and/or the mutual crosslinkage of SiOH groups of the vinyl polymer and also through the siloxane crosslinkage between the hydrolysable silyl group and/or the SiOH of the vinyl polymer and the SiOH group on the silica fine particles and by the action of the low expansibility of the silica fine particles. As a result, the primer composition becomes smaller in the expansion and shrinkage ascribed to the temperature difference than existing primers. Accordingly, neither cracking nor peeling off develops in the polysiloxane cured resin coated on the primer film surface over a long time.
However, it has been found that when the primer composition is subjected to a long-term weatherability test, performance reproducibility for use as a primer lowers with respect to the cracking and peeling off although depending on the ambient temperature or humidity during the course of coating and curing. Thus, a further improvement should be needed.